Typical methods proposed for producing polysilicon used as a raw material for a wafer for a semiconductor or photovoltaic power generation include a method for producing polysilicon called Siemens process. This Siemens process enables high purity polysilicon to be obtained, and is implemented as the most popular method for producing polysilicon.
FIG. 8 illustrates one example of a device for producing silicon by Siemens process. The device 50 for producing silicon includes a bottom panel 1 and a bell jar container 6. The bottom panel 1 is provided with metal electrodes 2 for energizing a silicon core wire 4, and the bell jar container 6 covers the bottom panel 1. In addition, a core wire holder 30 including an insertion hole for the silicon core wire 4 is fixed on the electrode 2 by, e.g., screws to hold and energize an end portion of the silicon core wire 4.
Then, in producing a polysilicon rod, both ends of the silicon core wire 4 formed in an inverted U shape are first fixed to the core wire holders 30. Next, the silicon core wire 4 is energized by the electrodes 2 through the core wire holders 30 and then heated to a silicon deposition temperature. Then, under this heat condition, silane compounds such as trichlorosilane or monosilane and reducing gas such as hydrogen are supplied into the reaction device to generate and deposit silicon on the silicon core wire 4, and then the silicon is collected as the silicon rod 5.
In addition, in recent years, in the method for producing the above polysilicon rod, some attempts have been made to produce a long silicon rod with a large diameter to obtain a large amount of silicon. Here, distortion and local load caused by expansion or shrinkage of such a large silicon rod increases at a silicon deposition stage or a cooling stage after the deposition. Thus, a leg portion of the silicon rod is broken (cracks appear), and consequently, destruction of the silicon rod 5 is caused.
One of the causes of the destruction of the silicon rod 5 is a stress generated, due to the cooling structure of the electrode 2, on the leg portion of the silicon rod 5, i.e., a contact portion between the silicon rod 5 (the silicon core wire 4) and the core wire holder 30.
More specifically, in the device 50 for producing silicon, the electrode 2 is generally made of metal such as SUS, copper, etc. To protect the electrode 2 from a high temperature atmosphere, provided is a cooling means (not shown) for water-cooling the inside of the device 50 for producing silicon. Consequently, the core wire holder 30 fixed to the electrode 2 is cooled, and also the temperature of the silicon core wire 4 inserted into the core wire holder 30 is reduced at the contact portion between the silicon core wire 4 and the core wire holder 30. Thus, in particular, when the silicon rod 5, which has been deposited by heating the silicon core wire 4 and is at a high temperature, is cooled and heat-shrinked immediately after the silicon deposition is completed, high stress is created in the contact portion between the silicon rod 5 and the core wire holder 30. Consequently, cracks appear in the core wire holder 30 or the silicon-deposited leg portion, and the destruction of the silicon rod 5 is caused
To solve the above problem, a method for temporarily raising and then reducing the temperature of the silicon rod is suggested to reduce the distortion in the silicon rod that causes generation of the stress (see, e.g., Patent Document 1). Then, such a method can reduce generation of the distortion in the silicon rod.
However, in recent years, a silicon rod with a larger diameter has been developed. Thus, the amount of distortion and load increase, resulting in an increase in the stress on the leg portion of the silicon rod. Thus, it is difficult to effectively reduce the cracks in the leg portion of the silicon rod and the destruction caused thereby with the method of Patent Document 1 only.
To solve the above problem, a method for using a core wire holder provided with annular pleats is suggested, and the method includes forming a core wire holder having a partially thin outer wall to reduce heat transfer and using an air space between the pleats as heat insulating portion to reduce heat conduction from the electrodes (see, e.g., Patent Document 2).